Photovoltaic DC/DC micro-converter

ABSTRACT

A photo-voltaic (PV) power generating system and a control system for PV array string-level control and PV modules serially-connected into strings of PV modules. The system includes plural parallel strings of serially-connected power-generating photovoltaic modules that form a PV array, DC/DC micro-converters that are coupled to a DC voltage buss and to the output of a corresponding photovoltaic module or to the output of a string of photovoltaic modules; a gating or central inverter; and a control system. The micro-converters are structured and arranged to include at least one of: an active clamp device, a ground fault detection device, and a fractional power converter that injects power in series or in parallel with voltage or current from the power-generating portion onto the DC buss.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/495,840 which was filed Jul. 1, 2009, now U.S.Pat. No. 8,106,537, and which claims the benefit under 35 U.S.C. §119(e)of U.S. Provisional Patent Application 61/133,634 filed on Jul. 1, 2008.Each of the aforementioned related patent and application is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

(Not applicable)

BACKGROUND OF THE INVENTION

A power converter for use with photovoltaic cells is disclosed, or, moreparticularly, individual DC/DC micro-converters for dedicated use withat least one photovoltaic module are disclosed.

As photovoltaic (PV) solar power installations continue to increase innumber and in scale, harvesting and managing power efficiently hasbecome more challenging. Equally as challenging is the management of PVpower installations on a national level via a “smart grid”. Inparticular, it is desirable to increase the demand for renewable energy,to supplement and/or replace energy produced via fossil fuels. EnhancingPV power use, however, requires reduction in the production cost perkilowatt hour and reduction in utility transaction costs for PVinterconnections.

For the latter, traditional PV power generating and control systems useat least one of centralized inverters, bipolar centralized inverters,string inverters, and micro-inverters. Conventionally, DC/AC invertershave been used to extract maximum power from PV systems that includearrays formed by plural PV modules connected in series and parallelconfigurations and to convert the unregulated generated DC power togrid-voltage, synchronized AC power. The AC power generated can betransmitted and distributed either directly to AC loads or throughdistribution transformers. According to this traditional approach,low-voltage DC power transfer concerns and simplicity of powerconversion options necessitate configuring the PV modules in serialstrings and/or in parallel string arrays. However, the deleteriouseffects of shading, soiling, and other lighting degradation onindividual PV modules and, hence, PV module characteristics matchingrequire greater consideration.

Referring to FIG. 1, for a photovoltaic array 10 to achieve its highestenergy yield and greatest efficiency, current practice includescarefully matching the electrical characteristics of each PV module 15in each series-connected string 12 and of each parallel-connected string14. Matching creates considerable labor and expense during manufactureat the factory. More problematically, even if PV modules 15 are ideallymatched at the time of manufacture, a single PV module 15 in any string12 can quickly degrade the performance, i.e., DC output, of the entirePV array 10. Indeed, decreasing the current or voltage output from asingle PV module 15 degrades the output of the entire string 12 ofseries-connected PV modules 15, which has a multiplied effect on theperformance of the entire PV array 10. This is especially true whendirect sunlight is blocked from all or some portion of one of more ofthe PV modules 15.

For example, if the amount or intensity of sunlight striking a discretePV module 15 is blocked, for example, due to shading, e.g., from clouds,vegetation, man-made structures, accumulated moisture, and the like, ordue to soiling, i.e., contamination with soil or other organic ornon-organic matter, then even ideally matched PV modules 15 performpoorly. Moreover, the affected PV module(s) 15 may suffer from excessiveheating.

When centralized inverters 13 are used, output from plural PV modules 15that are structured and arranged in strings 12 of parallel rows 14 ofstrings 12 is combined and processed. Power optimization andconditioning is, consequently, performed on the combined DC input.

Advantageously, these systems are highly evolved and reliable and,moreover, they facilitate centralized communication, control, andmanagement through the centralized inverter 13. Disadvantageously, thereis no PV string level management or control. Hence, overall arrayperformance is still adversely affected by underperforming individualstrings. Indeed, panel mismatch resulting from, inter alia, shading,soiling, and the like, reduces efficiency.

Traditionally, bypass diodes 16 and blocking diodes 18 are adapted todeal with the variability (matching) of discrete, individual PV modules15 and with solar irradiance. More specifically, to minimize degradationof the total DC output of the array 10 that may result from mismatch ordifferences in the voltage or current outputs of discrete PV modules 15,bypass diodes 16 can be integrated with each PV module 15. When forwardbiased, the bypass diodes 16 provide an alternate current path around anunderperforming PV module 15. Bypassing the underperforming PV module 15ensures that the string's 12 voltage and current outputs are not limitedby the voltage and current output of the underperforming PV module 15.Disadvantageously, bypassing the underperforming PV module 15 reducesthe string's 12 voltage output by, effectively, taking theunderperforming PV module 15 off-line.

Similarly, blocking diodes 18 can be integrated with strings ofseries-connected PV modules 12 in the PV array 10. When the totalvoltage output from a string of series-connected PV modules 12 exceeds abiasing voltage associated with the blocking diode 18, the DC voltageoutput is fed onto the DC bus for transmission to the inverter 13.However, if the total voltage output from the string of series-connectedPV modules 12 is less than the biasing voltage associated with theblocking diode 18, then the blocking diode 18 is not forward biased and,hence, voltage output from the string 12 is blocked from going the DCbus.

Bi-polar centralized inverters are slightly more efficient thanuni-polar centralized inverters. Advantageously, bi-polar applicationstend to be cheaper, lighter in weight, and do not suffer fromtransformer losses, simply because they do not include a transformer.Disadvantageously, as with centralized inverters, there is no PV stringlevel management or control, which, along with parallel processing andpanel mismatch, reduces efficiency. Bias voltages may also be introducedin the array. Furthermore, although the inverters themselves do notinclude transformer circuitry, a transformer is still required to stepup the power delivered to a commercial or utility grid.

To avoid reliance on bypass diodes 16 and blocking diodes 18, oneapproach has been to connect PV modules 15 to DC/AC micro-inverter(s).DC/AC micro-inverters are known to the art and embody the finest-grainedconfiguration in which maximum possible power can be extracted from eachPV module 15 regardless of mismatch, soiling, shading, and/or aging. Forthe purposes of this disclosure, “micro-inverters” will refer toinverters that perform a DC to AC power conversion and“micro-converters” (introduced below) will refer to converters thatperform a DC to DC power conversion.

Micro-inverters are adapted to reduce mismatch and other losses byconverting DC power to AC power locally, e.g., at each PV module 15 orcell and/or at every PV string 12 in the PV array 10, which facilitatesstring-level management. Micro-inverters have proven effective for smallsystems that yield higher total kilowatt hours (kWh). Disadvantageously,micro-inverters involve complex electronics that may requiresophisticated cooling. Moreover, large-scale applications may requireservicing and maintaining hundreds—if not thousands—of units, which havenot yet been engineered to operate dependably for 20 years or more.

Multi-phase AC systems also need to be configured from single phaseunits, requiring appropriate transformer step-up to utilization and/orto distribution voltages. Moreover, although generating single phase ACpower, the micro-inverter has double line frequency energy storagerequirements. This generally causes either a significant ripple currentthrough the PV module 15—which reduces yield—or requires utilization ofelectrolytic capacitors. Electrolytic capacitors, however, areunreliable and the acknowledged “Achilles heel” of any power conversionsystem that utilizes them.

Furthermore, integrating energy storage into a PV array 10 withmicro-inverters is not straightforward. For example, because the DC nodeis internal to each of the micro-inverters, each energy storage systemrequires a discrete, dedicated micro-inverter. The issue of gridinteraction and control can be daunting with so many devices inparallel.

Current practice needs with micro-inverters also include additionalelectronics, which normally are located in a hot environment, which isto say, on the reverse side (back) of the PV module 15. The ambientenvironment on the back of a PV module 15 is not particularly conduciveto long life of the electronics, having an operating range as high as80° C.

The challenges facing DC to DC micro-converter applications includeachieving a highly reliable, lower-installed cost per Watt system thatprovides increased kWh yields. Such systems should provide centralizedand de-centralized monitoring and control features; should includeelectronics that can be controlled locally or remotely, to react tovariable array and grid conditions; and that can be easily integratedwith a commercial or utility grid.

U.S. Pat. No. 6,127,621 to Simburger discloses a power sphere for aspinning satellite that purportedly minimizes mismatch losses on thesolar cells by providing individual DC/DC “regulators” for eachindividual solar cell, to regulate the power delivered to a load. U.S.Pat. No. 6,966,184 to Toyomura, et al. discloses a PV power-generatingapparatus having power conversion devices individually connected tosolar cell elements to convert the output of the elements. The pluralDC/DC converters are connected in parallel and are operated so thatchanges in the input voltage to a DC/AC inverter move the operatingpoint of the solar cell element, which changes the input voltage to theDC/DC converters. In this manner, input voltage to the DC/AC inverterfrom each converter is controlled to be the same.

U.S. Pat. No. 7,193,872 to Siri discloses a power supply having aninverter for connecting plural DC power sources to a utility grid usinga single DC/DC conversion stage. The Siri system purports to controlcurrent based on feed-forward compensation as some function of an inputpower commanding voltage (V_(ERR)) More specifically, the current andvoltage from a solar array are sampled from which the input powercommanding voltage is output. A current reference generator generates areference current (I_(REF)) which is the product of the input powercommanding voltage, an instantaneous utility line voltage, and theinverted square of the V_(RMS) signal.

A photovoltaic power system that includes plural photovoltaic strings oran array of power-generating photovoltaic modules and a controllertherefor that provide PV string level control, to regulate and stabilizeoutput voltage of each PV string individually, to harvest greater energyand increase kWh produced is desirable.

Means for integrating replacement modules into a PV array without havingto match the electrical properties of the replacement module to those ofthe modules already in the array is also very desirable.

SUMMARY OF THE INVENTION

A photovoltaic power generating system is disclosed. The systemcomprises a string or array of power-generating photovoltaic modules; aplurality of micro-converters, each of which is coupled to a DC voltagebuss and to the output of a discrete photovoltaic module of the stringor array of power-generating photovoltaic modules; and a gating inverterthat is structured and arranged to provide AC power to a grid; orequivalently a gating DC/DC converter that is coupled to a high voltageDC buss for industrial DC power supply applications, e.g., DC powersupplies for chlor-alkali or copper-winding electrochemical processes.

The photovoltaic power system is structured and arranged to maximizedesign flexibility, which leads to enhanced longevity. For example,different panel technologies, vintages, sizes, mounts, and manufacturingbrands can be incorporated into the same array, which can be efficientlycontrolled by the disclosed invention. The range of power ratings forthe disclosed system is between 30 kW and 1 MW.

Managerial benefits include increased visibility which includes in-depthdiagnostic and performance information, enabling the conditions of PVstrings and/or corresponding DC/DC converters to be monitored remotely.As a consequence, poorly performing or malfunctioning PV strings or PVstrings having ground faults can be systematically isolated withoutinterrupting throughput from the remaining PV strings.

From a performance standpoint, both energy throughput and return oninvestment (ROI) can be increased significantly. Indeed, eliminatinglosses that otherwise would occur when outputs from PV string inparallel are combined and processed reduces the cost per kWh through thelifespan of the PV array. An increase in output of between 5 and 20percent is predicted.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by referring to the DetailedDescription of the Invention in conjunction with the Drawings, of which:

FIG. 1 shows an array of series- and parallel-connected photovoltaicpower modules according to the prior art;

FIG. 2 shows an array of series- and parallel-connected strings ofphotovoltaic power modules with micro-converters according to thepresent invention;

FIG. 3 shows a gating inverter in combination with a DC/DC photovoltaicsystem according to the present invention;

FIGS. 4A-4C show an isometric view of DC/DC photovoltaic system (4A) andinsets showing coupling of plural PV strings to the micro-converter (4B)and coupling of plural micro-converters to a local controller (4C)within a string combiner according to the present invention;

FIG. 5 shows representative current-voltage curves for three PV stringsof PV modules;

FIG. 6 shows an array of PV string-based micro-converters having anactive clamp device in accordance with the present invention;

FIG. 7 shows voltage-current curves characteristic of solar PV stringsfor warm and cold operating conditions;

FIG. 8 shows a diagrammatic of a ground fault condition in a PVstring-based micro-converter;

FIG. 9 shows a block diagram of a ground fault detecting deviceintegrated into a micro-converter;

FIG. 10 shows a micro-converter and a voltage injection-based fractionalpower converter in parallel with the power source for injecting voltagein series with the power source; and

FIG. 11 shows a micro-converter and a current injection-based fractionalpower converter in series with the power source for injecting current inparallel with the power source.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 2-5 a photovoltaic (PV) system controlled at thestring-level by plural DC/DC micro-converters will now be described. ThePV system 29 and, more specifically, the control system 21 for the PVsystem 29 is structured and arranged to extract maximum individualstring power (hereinafter, the “Maximum Power Point” or “MPP”) from eachof the PV modules 22 in each string 25 of serial-connected PV modulesthat make up the power-generating portion of the PV system 29.

The desirability of string-level control is shown illustratively in FIG.5, which shows current-voltage curves 52, 54, and 56 for three discretestrings of PV modules. Each of the curves 52, 54, and 56 includes an MPP55 at some location on the curves. The MPP 55 refers to the point ofmaximum power for an entire string 25 of PV modules 22.

Because of expected mismatch between PV strings 25, the correspondingMPPs 55 for each string 25 occur or may occur at different currentsand/or at different voltages for each PV string 25. As a result, thecontroller 21 is adapted to regulate and to stabilize output voltagefrom each PV string 25 at each MPP 55, to harvest greater energy andincrease kWh produced.

Were the controller 21, instead, adapted to regulate output current oroutput voltage using a fixed, predetermined voltage or a fixed,predetermined current, which is shown illustratively in FIG. 5 atcurrent I_(o) and voltage V_(o), the MPP 55 for each PV string 25 may bemissed, which means less energy and fewer kWh produced.

The PV System

The PV system 29 includes a power-generating portion, a power controland distribution portion, and the aforementioned control system. In thediscussion below, those of ordinary skill in the art can appreciate thatelements described as structure for the power-generating portion could,instead, be included as elements for the power control and distributionportion or the control system, and vice versa. For example, the MPPcontroller 21, the control unit 24 for the gating inverter 28, and thecentral or micro-grid controller 34, which are described below asseparate and distinct elements of the power-generating portion, thepower control and distribution portion, and the control system,respectively, could, instead, be included as elements in a singlecontrol structure for exercising control over all aspects and operationof the PV system 29.

In general terms, referring to FIG. 2, the power-generating portion ofthe PV system 29 includes plural parallel-connected PV strings 25 of PVmodules 22 that collectively form a PV array. PV modules 22 are wellknown to the art and will not be described in detail. The PV arraycorresponds to multiple PV strings 25 that are electrically disposed inparallel so that the output of each PV module 22 and each PV string 25is delivered to a common buss. The power control and distributionportion includes plural DC/DC power converters 20 each having a localMPP controller 21, and a large, distribution substation-class,grid-connected (gating) inverter 28 having a control unit 24.

The PV system 29, and, more particularly, the control system isstructured and arranged to ensure that any PV module(s) 22 thatbecome(s) shaded from direct sunlight, that become(s) contaminated withdirt or grime, and/or that are otherwise covered with or by some foreignmatter on all or on any portion of the PV module(s) 22 does not cause anentire PV string 25 of the PV array or multiple strings 25 in the array,to operate at less than maximum power transfer efficiency, i.e., outsideof MPP. In short, rather than allowing a single PV module 22 or a fewaffected PV modules 22 to diminish power generation of the entire PVarray, the control system is structured and arranged to temporarilyprevent the affected PV module(s) 22 from delivering power to thevoltage buss 27 until such time as the cause of the affectation has beencorrected.

Referring to FIG. 2, a serial PV string 25 having plural PV modules 22that are electrically disposed in parallel is shown. Advantageously, adedicated power converter 20 (hereinafter “micro-converter”) is coupledto the output of each PV module 22 and/or to each PV string 25 of PVmodules 22. The micro-converter 20 can be physically mounted on thereverse side (back) of the discrete PV module 22 that it controls.Alternatively, as shown in FIG. 4A-4C, discrete micro-converters 20 thatare electrically coupled to corresponding PV modules 22 or PV strings 25of PV modules 22 can be centralized in a control box 35, such as theSolstice™ system manufactured by Satcon Technology Corporation ofBoston, Mass. The Satcon control box 35 is a natural replacement forcombiner boxes or “smart” combiner boxes, which are in common use inpopular PV systems today.

The micro-converters 20 are adapted to receive the input from that PVmodule 22 and/or from an entire string 25 of PV modules 22. Morespecifically, each micro-converter 20 is adapted to receive electricaloperating parameters, e.g., terminal current, terminal voltage, power,and the like, from a corresponding PV module 22 and/or from an entirestring 25 of PV modules 22. Optionally or alternatively, themicro-converter 20 can also be adapted to receive thermal operatingparameters from the associated PV module 22. Each micro-converter 20 isadapted to communicate these operating parameters to the control unit 24of the gating inverter 28 (or converter) and/or to a remote, centralcontroller 34. The means of communicating such data can include usingpowerline carrier communication, a wireless connection, and so forth.

This communications capability allows each micro-converter 20 to sense acurrent level and/or voltage level generated by each PV module 22 and/orstring 25 of PV modules 22 and to communicate with the control unit 24of the gating inverter 28 and/or to communicate remotely with thecentral controller 34, to provide PV module 22 status signals. As aresult, the control unit 24 of the gating inverter 28 and/or the remotecentral controller 34 can communicate with an associated EnergyManagement System or utility or microgrid system controller, to providefine-grained information on the performance of the PV modules 22, the PVstrings 25, and the entire PV array.

The topology of the micro-converter 20 can include (for example and notfor the purposes of limitation): an interleaved boost converter, aninterleaved flyback converter, an interleaved forward converter, anH-bridge converter, a multi-stage converter, isolated converters,non-isolated converters, and the like. To allow air circulation andconvectional cooling, the micro-converter 20 can stand off the back ofthe corresponding PV module 22 if located at the module 22.

Advantageously, when micro-converters 20 are disposed on the back of thePV module 22 itself or are contained in a string combiner 35 (FIG. 4Aand FIG. 4B), energy storage at the PV module 22 is not necessary.Instead, the energy or power generated by each individual PV module 22and each PV string 25 is introduced to and collected on a high voltageDC (HVDC) buss 27. Because the HVDC buss 27 carries higher voltage, theassociated currents are relatively low, which reduces conduction losses.Manufacturing costs associated with conductive materials are alsoreduced. Within the limitations of the regulatory environment, e.g., ULor CE, the higher the voltage, the more compact and economic the system.

Referring to FIG. 2 and FIG. 3, the HVDC buss 27 is electrically coupledto a large, distribution substation-class, grid-connected (gating)inverter 28, the output of which is delivered to a commercial grid, to autility grid 26 and/or to a local AC load. The gating inverter 28includes an optional energy storage device 23 and a control unit 24,which can be organic to the inverter 28 or (as shown in FIG. 4C) can beelectrically coupled to a local MPP controller 21 that is/are disposedproximate to the micro-converters 20.

DC storage of power is easily integrated with this approach and willpermit different sizings of PV modules 22, PV strings 25, and gatinginverters 28 so that the array sizing can be infinitely fine-grainedwithout requiring peak power capability to be matched by the gatinginverter 28. Elimination of the need for energy storage at or near eachindividual PV module 22 (such as the double frequency requirement of themicroinverter) and integrating an energy storage device 23 with thegating inverter 28, eliminates one of the biggest problems ofmicro-inverter-based or micro-converter-based systems.

Energy storage is easily integrated using, for example, bi-directionalconverters operating from the HVDC buss 27 and/or “AC storage” 23coupled at the output stage. “AC storage” 23 refers to a battery orother DC energy storage device in combination with a separate DC/ACinverter. In addition to readily accommodating energy storage, thisarchitecture is also amenable to supplying relatively large DC loadsand/or AC loads locally, such as for facility AC loads or industrial DClighting, and is also compatible with modern micro-grid infrastructureswhere applicable. The industrial DC buss can be fed directly by theregulated dc buss of this micro-converter architecture.

Referring to FIG. 3, at the input stage of the gating inverter 28, aswitching system 32 controls application of power generated by the PVarray to the gating inverter 28. A DC power surge protector 31 iscoupled to the HVDC buss 27 at or proximate to the switching system 32.At the output stage, to guard against AC back feed from thecommercial/utility grid 26, an AC surge protector 33 is provided. The ACsurge protector 33 can include rectifier diodes that serve as blockingdiodes.

A commercial- or utility-scale grid 26 can generate standard voltages(480V or 600V) directly from the HVDC buss 27 without a transformer.Accordingly, optionally, the PV system 20 would not require a 60-Hztransformer, which is to say that the gating inverter 28 can betransformer-less.

Recalling that each micro-converter 20 is adapted to provide operationaldata, e.g., current, voltage, power, and the like, and, optionally,other data, e.g., temperature and the like, about its corresponding PVmodule(s) 22, the local MPP controller 21 and the control unit 24 of thegating inverter 28 are adapted to provide such operational data to aremote central controller 34. Awareness of each PV modules' operatingparameters allows the central controller 34 to adjust the parameters ofthe boost circuit of the micro-converters 20 to maintain the chosenoutput voltage, or to disconnect select PV modules 22 or strings 25 fromthe PV array when a desired output cannot be maintained due todegradation of output. Advantageously, the ability to isolate or removeaffected, low-output PV modules 22 or strings 25 prevents degrading theefficiency of the entire PV array.

The MPP controller 21, i.e., the ultra-local controller within themicro-converter 20, and the control unit 24 coupled to the gatinginverter 28 constitute components of a central control system, which caninclude hardware and software applications. The control system isadapted to extract MPP from each individual PV module 22 and from eachstring 25; to selectively use all or less than all of the PV modules 22or strings 25 at any given time for power generation; to instrument eachPV module 22 for power, voltage, current, temperature, and othercharacteristics to achieve MPP; and to integrate energy storage fully.These applications and more can be accomplished by the MPP controller21, by the control unit 24, and by the remote central controller 34.

The central controller 34 can also be coupled to the Internet to providefor Web-based monitoring using, for example Web-based management toolssuch as PV ZONE™ and PV VIEW™, which are provided by Satcon TechnologyCorporation. PV VIEW™ is a Web-enabled data monitoring system that isadapted to monitor power inverters. PV ZONE™ is a Web-based sub-arraymonitoring program that monitors solar radiation, module temperature,ambient temperature, wind speed, wind direction, and the like.

By connecting an input/output device to the Internet via a local areanet (LAN), wide area net (WAN), cellular modem, and the like, the PVsystem 29 can provide complete real-time performance data of eachmicro-converter. PV VIEW™ and PV ZONE™ can include a variety of optionalenvironmental and weather station capabilities. This information can bestored and processed on a remote Web server and can be made availablefrom anywhere on the Web to anywhere on the Web through the PV VIEW™ Webportal.

Such Web-based tools enable users, inter alia, to meter the output ofthe PV system 29, e.g., kWh of operation, and to monitor meteorologicalinformation to ascertain the existence of favorable or unfavorablemeteorological conditions. As a result, the control system 34 provides adegree of energy management that includes string-level or module-levelcontrol using micro-converters 20, which enables PV systems 29 toharvest more energy and increase the kWh produced during a fixed periodof time. Monitoring and control tools for commercial PV systems areprovided by many third party providers. These systems are readilyadapted to work with all such providers as well as with utility SCADAsystems. SCADA (Supervisory Control and Data Acquisition) refers to atypically-centralized system that monitors and remotely controls aspectsof power production at an entire site, at a combination of sites or at acomplex that covers a relatively large area. SCADA is a proprietary,non-Web-based control system.

Advantageously, when using micro-converters 20, individual PV modules 22can fail; however, any such failure impacts the total system power onlymarginally. Replacement of failed PV modules 22 can be achieved usingany suitable PV module 22 of any suitable technology, eliminatingrequirements for exact matching as to age, size, manufacturer, and soforth, and PV system 20 downtime.

The micro-converter 20 is adapted to accommodate a full range ofvoltages from its corresponding PV module 22, which, currently, istypically between approximately 10 VDC and approximately 150 VDC. Forexample, a micro-converter 20 can boost module output from 48 VDC(typical) to a fixed voltage nominally of 550 to 1200 VDC, furtherallowing for significantly lowered interconnect currents andestablishing a standard for the interconnect that allows the gatinginverter 28 design to be simplified and, if desired, interconnectiontransformers to be eliminated. Lower interconnect current reducesinterconnect transmission losses and allows for smaller, less expensiveinterconnect conductors.

Comparing the currently disclosed DC/DC micro-converter concept with theDC/AC micro-inverter concept, the fundamental issues are those of DCpower versus AC power and high voltage versus low voltage. For example,with AC, the ratio of RMS-to-average value is 0.707V/0.636V or about1.11. As a result, the conduction losses associated with an AC are 1.11squared, or 23% higher than for a DC system. This result, however,assumes identical busswork, when, in reality, the AC system has 40%higher voltage stress than an equivalent DC system, making a DC systemeven more preferable.

Another advantage of AC versus DC has to do with skin depth. Normally,buss bars for heavy AC current are rarely more than ½ inch (12 mm) indiameter except when mechanical reasons dictate using a larger buss bar.As current magnitudes increase, which is certainly the case in largecommercial PV systems, and line frequency remains relatively low (60Hz), a wire radius larger than ⅓^(rd) of an inch (8 mm) could be used toadvantage to reduce conduction losses. With a skin depth in copper of 6mm. and a skin depth in aluminum of approximately 8 mm., there is littlevalue in using conductor cables that are more than ½-inch in diameter.

The final points of comparison between micro-inverter andmicro-converter approaches relates to the reduced electronics anddramatic reduction in energy storage requirements for themicro-converter. The requirement for double line frequency energystorage and location of the micro-inverter proximate to the module meanthat even well-built, initially-reliable units are liable to suffer fromwear and tear after several years have passed due to the use ofelectrolytic capacitors.

Integrated Active Clamp Device

Elevated voltage at the input terminals of a DC-DC micro-converter,e.g., from a PV solar array, may exceed the maximum or breakdown voltage(V_(br)) of semiconductor devices, e.g., MOSFETs, IGBTs, and so forth,used in the micro-converter, damaging, and likely destroying, thesedevices. These deleterious elevated voltages may be caused by atransient condition or, often, may be due to extremely cold conditionsthat drive up the solar PV array open-circuit voltage (V_(oc1) orV_(oc2)). For example, referring to FIG. 6, while the PV system 70 is inoperation, an event on either the dc side 72 or the grid side 71 of theinverter 75 may cause the system 70 as a whole to trip off line. If theambient conditions are sufficiently cold, the situation described canreadily develop. Indeed, when the sun begins to shine on cold,previously-dormant PV panels in a PV string 63, there is very littlecurrent flow, but voltage being produced by the PV strings 63 isrelatively high.

Complicating the voltage increase at the PV strings 63, when cold, thebreakdown voltage (V_(br)) of various semiconductor devices in themicro-converter 60 decreases, which produces a highly disadvantageouscondition. Indeed, if the open-circuit voltage (V_(oc)) from the PVstrings 63 passing through the semiconductor devices of amicro-converter 60 exceeds the breakdown voltage (V_(br)) of thosesemiconductor devices, the semiconductor devices can be damaged ordestroyed, which also may cause collateral damage.

This is shown schematically in FIG. 7, which shows representativerelative voltage-current characteristics of a PV panel or PV string 63in cold and warm operating temperatures. The voltage-current curvesinclude maximum power points (V_(mp), I_(mp)) for each condition.

Those of ordinary skill in the art can appreciate that converters 60 arenormally sized for peak current from the PV string 63 and, moreover,that the PV string 63 is, in reality, a current-limited source. As aresult, when a high-voltage condition develops, which is to say that, ininstances in which the PV string 63 operating point (V_(mp), I_(mp)) or,more likely, the open-circuit voltage (V_(oc1) or V_(oc2)) approachesthe maximum voltage (V_(br)) of the semiconductor devices, turning ONsemiconductor switching devices 65 in each micro-converter 60controlling a corresponding PV string 63 advantageously creates ashort-circuit. The effect of shorting the micro-converters 60 is toclamp the PV array or PV string 63 voltage at or substantially at zero.

The zero voltage condition in FIG. 7 is labeled as I_(sc). I_(sc) refersto the short-circuit current of the PV string 63. During a zero voltagecondition the operating voltage is zero so the likelihood of damaging ordestroying the semiconductor devices is nil.

Advantageously, during zero voltage clamping, the PV strings 63 areloaded, causing each PV string 63 inherently to heat up. As the PVstrings 63 heat up, the open-circuit voltage (V_(oc2)) decreases,permitting a subsequent power up of the PV string 63 at lower voltages.Furthermore, the breakdown voltage (V_(br)) of the semiconductor devicesincreases as they warm up. When the semiconductor devices aresufficiently warm, the breakdown voltage (V_(br)) is greater than theopen-circuit voltage (V_(oc1)) and the semiconductor switching devices65 can be turned OFF.

Accordingly, the micro-converter 60 and system 70 described hereinabovecan be improved by integrating an active clamp device 69 into eachmicro-converter 60. Clamp devices and their uses are well-known to theart and need not be described in great detail. The integrated activeclamp device 69 of the present invention includes a voltage sensingdevice, a comparator, and a control device, which could be the controlsystem for the entire micro-converter or a separate controller just forthe active clamp device 69.

A voltage sensing device (not shown) is adapted to measure voltage oneach PV string 63. The voltage sensing device outputs a voltage signalcommensurate with the measured voltage to a comparator (not shown). Thecomparator compares the output voltage signal to a pre-establishedthreshold voltage that is less than or equal to the limiting breakdownvoltage (V_(br)) of any of the semiconductors devices integrated intothe micro-converter 60.

When the output voltage signal equals the pre-established thresholdvoltage, the control device (not shown) of the active clamp device 69automatically and instantly turns ON the semiconductor switchingdevice(s) 65 of each micro-controller 60 at which the measured voltageequals the pre-established threshold voltage and/or the breakdownvoltage (V_(br)) of the semiconductors devices. This creates a zerovoltage condition.

Ground Fault Deduction from Apparent Power Measurements/Comparisons

As the use and popularity of solar power has increased, the occurrenceof numerous fires at various solar energy sources has caused extensivedamage to the sources themselves and to related ancillary structuresand, furthermore, has hindered the rate of expansion of the use of solarpower for industrial and residential applications. In many instances,the fires were caused by ground or stray current problems, oftenresulting from shoddy construction in which local heating of wiring hasprogressively created major fires.

To avoid such events, the National Electric Code (the “Code”) requiresthat when an inverter recognizes a ground fault, the inverter mustdisconnect the source from the utility grid or micro-grid. Then, thecause of the flowing current condition, which is evidence of the groundfault, must be ascertained and repaired before the inverter re-connectsthe power source to the grid or micro-grid. Accordingly, themicro-converter and system described hereinabove can be improved byproviding means for detecting or deducing a ground fault condition ateach micro-converter, which is to say at the PV string level, so thatstrings 63 and not the entire system 70 are off-line.

Referring to FIG. 8, because there is minimal energy storage within themicro-converter 60, for time scales longer than several switchingperiods, input power from the PV string 63 equals or essentially equalsthe output power from the micro-converter 60. Mathematically,V _(string) ·I _(in) =V _(dc) ·I _(out)in which V_(string) is the voltage from the corresponding PV string 63,V_(dc) is the output voltage to the inverter 75, and the currents I_(in)and I_(out) are each measured, respectively, on leads 67 and 68, whichcorrespond to the input of the micro-converter 60 and the output of themicro-converter 60, respectively.

If the relationship does not hold true, there is only a finite number ofexplanations. For example, either some of the sensing elements arefaulty or, more likely, an additional current I_(gnd), i.e., a stray orground current, is being superimposed on the micro-converter wiring.This additional current I_(gnd) adds to one of either the input oroutput variables and subtracts from the other and, most likely, isindicative of a ground fault current.

As a result, by identifying an apparent power mismatch, micro-converters60 can be used as a ground fault detector at the PV string level ratherthan at the system level. Advantageously, only a single PV string 63rather than the entire system would have to be disconnected from theinverter and the grid or micro-grid. As a result, the micro-converter 60for each PV string 63 can be adapted to measure all of these input andoutput variables/values. The measured variables/values can then bemanipulated to verify that input power equals output power.

In its simplest embodiment, referring to FIG. 9, the ground faultdetector 90 can be a separate device or can be integrated into theimproved micro-converter 60. The ground fault detector 90 includes acontrol or processing device 95 that is structured and arranged toreceive measured voltage signals 98 and 99 from voltage measurementdevices 91 and 92 and to receive measured current signals 96 and 97 fromcurrent measurement devices 93 and 94, which are disposed on the outputnegative lead 67 and input negative lead 67. The output current sensor93 could be installed on either the negative output lead 68 or thepositive output lead 108. The input current sensor 94 could be installedon either the negative input lead 67 or the positive input lead 107. Thecontrol or processing device 95 could be a stand alone processor or theprocessing function can be performed on any control system describedhereinabove.

The processing device 95 can be hardwired or can include an application,drive program, software, and the like to calculate input power andoutput power using the formula above. The control or processing device95 also includes a comparator function that can be used to compare inputpower to output power. The comparator is adapted to provide a signal tothe control or processing device 95 when the comparison of the powersindicates a ground fault current. The processing device 95 is furtheradapted to disconnect the particular PV string 63 from the voltage busswhen there is a power mismatch as evidenced by an inequality of inputpower and output power. Those PV strings 63 at which there is noindication of a ground fault condition remain connected to the inverter75 and the inverter 75 remains connected to the utility grid ormicro-grid.

Fractional Power Converter

Another traditional use of converters is to harvest energy from PV powersources by converting or processing power at its native voltage to analternative, constant voltage level for the load. Say, for example, a PVstring provides 2.9 kW of power to the converter. Under conventionalpractice, the converter should be sized to process 2.9 kW of power plusor minus ten percent (±10%) to account for changes and variations fromthe PV string. The efficiency of the converter represents the fractionof energy successfully converted from the PV source to the load, whichis to say that power that is not lost.

An alternative means of harvesting energy is to allow the power sourceto deliver some of its energy directly to the load while processing onlya relatively small portion of the source power to supplement that whichis delivered directly. Advantageously, for the power that is delivereddirectly, there is no need to process the power and there is no energylost to the energy harvesting converter. Hence, processing can bereduced from 100 percent±10% to just the ±10%. This is referred to aspartial power processing, which can be practiced using a fractionalpower converter.

FIGS. 10 and 11 show embodiments of energy harvesting circuits 80 a, 80b that are structured and arranged to convert, respectively, voltage orcurrent to required or desired levels for the load 85. Such harvestingis made possible by converting only a fraction of the total power. Withthese arrangements, the PV source 81 delivers a majority of the totalpower directly to the load 85 while a smaller fraction of the power isprocessed. This is achieved using either a voltage-injection fractionalpower converter (FPC) approach or a current-injection FPC approach.Advantageously, FPCs perform partial power processing, which, in largepart, results from cell, panel, string, and or array mismatch, at the PVcell or PV module level.

FIG. 10 shows a voltage injection-type FPC system 80 a that, for thepurpose of illustration and not limitation, is based on a flybackconverter. Those of ordinary skill in the art can appreciate thatconverter types for the voltage injection-type FPC system 80 a otherthan a flyback converter are possible. The micro-converter 60 includesan FPC 82 that is structured and arranged to convert PV source voltageto a higher level for the load 85. For this purpose, as shown in FIG.10, the FPC 82 with output 87 is in series with the PV source 81 and theFPC 82 is adapted to draw a fraction of the total power from the PVsource 81 through a parallel input port 83, and to inject a voltage(V_(FPC)) in series with voltage (V_(IN)) directly from the PV (voltage)source 81. The injected voltage provides the additional voltagenecessary to meet required or desired amounts.

FIG. 11 shows a current injection-type FPC systems 80 b that, also, forthe purpose of illustration and not limitation, is based on a flybackconverter. Here again, those of ordinary skill in the art can appreciatethat converter types for the current injection-type FPC system 80 bother than flyback converters are possible. The FPC 84 is structured andarranged to convert PV source voltage to a lower level for the load 85.More particularly, the FPC 84 is adapted to draw a fraction of the totalpower from the PV source 81 through a series input port 86, and toinject current (I_(FPC)) in parallel with the output current (I_(PV))directly from the PV source 81.

Optionally, these FPCs can be bi-directional (not shown), which willprovide the plus-or-minus 10%. However, this requires additional activesemiconductor devices. Advantageously, bi-directional converters have asingle circuit that is capable of injecting power in series (as voltage)or in parallel (as current) while being powered from the other port.This approach can be applied at any level, from the array, to thesub-array, to the string, to the module, and even down to the celllevel, where, by comparison, smaller corrections are necessary.

Many changes in the details, materials, and arrangement of parts andsteps, herein described and illustrated, can be made by those skilled inthe art in light of teachings contained hereinabove. Accordingly, itwill be understood that the following claims are not to be limited tothe embodiments disclosed herein and can include practices other thanthose specifically described, and are to be interpreted as broadly asallowed under the law.

What we claim is:
 1. A photovoltaic power generating system comprising:a power-generating portion including plural parallel strings ofpower-generating photovoltaic modules, the power-generating photovoltaicmodules being coupled in series in each of the parallel strings, eachstring of the plural strings and each of the modules having an output; aplurality of DC/DC micro-converters, each micro-converter of theplurality of DC/DC micro-converters being coupled to a regulated DCvoltage buss and to the output of a corresponding photovoltaic module orto the output of a corresponding string of photovoltaic modules, whereineach micro-converter of said plurality of micro-converters includes: atleast one semiconductor device having a breakdown voltage, at least onesemiconductor switching device, and an active clamp device; an inverterthat is coupled to the regulated DC voltage buss and to a load; and acontrol system that is structured and arranged to control each string ofphotovoltaic modules.
 2. The photovoltaic power generating system asrecited in claim 1, wherein the active clamp device includes: a voltagesensing device that is adapted to measure a voltage on a correspondingstring of photovoltaic modules and to generate a voltage signalcommensurate with the measured voltage; a comparator that is adapted toreceive the voltage signal from the voltage sensing device and togenerate an output signal when said output signal equals at least one ofa pre-established threshold voltage and the breakdown voltage of the atleast one semiconductor device; and a controller that, after receivingthe output signal from the comparator, is adapted to create a zerovoltage condition in the at least one semiconductor switching device inthe corresponding micro-converter.
 3. The photovoltaic power generatingsystem as recited in claim 2, wherein the controller is adapted to turnoff the at least one semiconductor switching device when said controllerno longer receives the output signal from the comparator.
 4. A method ofcreating a zero voltage condition in the photovoltaic power generatingsystem recited in claim 1, the method comprising: measuring a voltage ofeach string of photovoltaic modules; generating a voltage signalcommensurate with the measured voltage of each string of photovoltaicmodules; comparing the voltage signal to at least one of apre-established threshold voltage and the breakdown voltage of the atleast one semiconductor device; generating an output signal when thevoltage signal equals at least one of a pre-established thresholdvoltage and the breakdown voltage of the at least one semiconductordevice; and creating a zero voltage condition in the at least onesemiconductor switching device.
 5. A control system for a photovoltaicpower-generating system that includes a power-generating portionincluding plural parallel strings of power-generating photovoltaicmodules, each string of the plural strings and each of the moduleshaving an output, a plurality of DC/DC micro-converters, eachmicro-converter of said plurality of DC/DC micro-converters beingcoupled to a voltage buss and to the output of a correspondingphotovoltaic module or to the output of a corresponding string ofphotovoltaic modules, wherein each micro-converter of said plurality ofmicro-converters includes: at least one semiconductor device that has abreakdown voltage, at least one semiconductor switching device, and anactive clamp device; an inverter that is coupled to the voltage buss andthat is structured and arranged to provide power to a grid or to a DCload, the control system being structured and arranged to control eachstring of photovoltaic modules and to create a zero voltage condition inat least one semiconductor switching device of a corresponding DC/DCmicro-converter.
 6. The control system as recited in claim 5, whereinthe active clamp device includes: a voltage sensing device that isadapted to measure a voltage on a corresponding string of photovoltaicmodules and to generate a voltage signal commensurate with the measuredvoltage; a comparator that is adapted to receive the voltage signal fromthe voltage sensing device and to generate an output signal when saidoutput signal equals or substantially equals at least one of apre-established threshold voltage and the breakdown voltage of the atleast one semiconductor device; and a controller that, after receivingthe output signal from the comparator, is adapted to create a zerovoltage condition in the at least one semiconductor switching device inthe corresponding DC/DC micro-converter.
 7. The control system asrecited in claim 6, wherein the controller is adapted to turn off the atleast one semiconductor switching device when said controller no longerreceives the output signal from the comparator.
 8. A method of detectinga ground fault condition at a photovoltaic power generating systemhaving a power-generating portion including plural parallel strings ofpower-generating photovoltaic modules, the power-generating photovoltaicmodules being coupled in series in each string of the plural parallelstrings, each of the strings and each of the modules having an outputand the power-generating portion is structured and arranged to deliversome energy directly to a load; a plurality of DC/DC micro-converters,each micro-converter of said plurality of DC/DC micro-converters beingcoupled to a regulated DC voltage buss and to the output of acorresponding photovoltaic module or to the output of a correspondingstring of photovoltaic modules; an inverter that is coupled to theregulated DC voltage buss and to the load; and a control system that isstructured and arranged to control each string of photovoltaic modules,the method comprising: measuring a voltage level at each string(V_(string)); measuring a voltage level on the regulated DC voltage buss(V_(DC)); measuring a current level on a negative or a positive outputlead of the regulated DC voltage buss coupled to a correspondingmicro-converter (I_(out)); measuring a current level on a negative or apositive input lead of the regulated DC voltage bus coupled to thecorresponding micro-converter (I_(in)); calculating a power into thecorresponding micro-converter and a power out of said correspondingmicro-converter; comparing the power into the correspondingmicro-converter to the power out of said corresponding micro-converter;and providing a signal to the control system when the power into thecorresponding micro-converter does not equal or substantially equal thepower out of said corresponding micro-converter.
 9. The method asrecited in claim 8 further comprising disconnecting the correspondingstring from the regulated DC voltage buss when the power into thecorresponding micro-converter does not equal the power out of saidcorresponding micro-converter.
 10. The method as recited in claim 8,wherein comparing the power into the corresponding micro-converter tothe power out of said corresponding micro-converter uses the followingformula:V _(string) ·I _(in) =V _(dc) ·I _(out).
 11. A photovoltaic powergenerating system comprising: a power-generating portion includingplural parallel strings of power-generating photovoltaic modules, thepower-generating photovoltaic modules being coupled in series in eachstring of the parallel strings, each of the strings and each of themodules having an output, wherein the power-generating portion isstructured and arranged to deliver some energy directly to a load; aplurality of DC/DC micro-converters, each micro-converter of saidplurality of DC/DC micro-converters being coupled to a regulated DCvoltage buss and to the output of a corresponding photovoltaic module orto the output of a corresponding string of photovoltaic modules, whereineach micro-converter includes: at least one voltage sensing deviceadapted to measure a voltage level at each corresponding string(V_(string)), at least one voltage sensing device adapted to measure avoltage level on the regulated DC voltage buss (V_(DC)), at least onecurrent sensing device adapted to measure a current level on a positiveor a negative output lead of the regulated DC voltage buss coupled tothe corresponding micro-converter (I_(out)), at least one currentsensing device adapted to measure a current level on a positive or anegative input lead of the regulated DC voltage bus coupled to thecorresponding micro-converter (I_(in)), and a control device that isadapted to calculate a power into the corresponding micro-converter anda power out of said corresponding micro-converter and to compare thepower into the corresponding micro-converter to the power out of saidcorresponding micro-converter; an inverter that is coupled to theregulated DC voltage buss and to the load; and a control system that isstructured and arranged to control each string of photovoltaic modules.12. The photovoltaic power generating system as recited in claim 11,wherein the control device of each micro-converter is further adapted todisconnect the corresponding string from the DC voltage buss when thepower into the corresponding micro-converter does not equal orsubstantially equal the power out of said corresponding micro-converter.13. A control system for a photovoltaic power-generating system thatincludes a power-generating portion including plural parallel strings ofpower-generating photovoltaic modules, each string of the plural stringsand each of the modules having an output; a plurality of DC/DCmicro-converters, each micro-converter of said plurality of DC/DCmicro-converters being coupled to a voltage buss and to the output of acorresponding photovoltaic module or to the output of a correspondingstring of photovoltaic modules, wherein each micro-converter of saidplurality of micro-converters includes: at least one voltage sensingdevice adapted to measure a voltage level at each corresponding string(V_(string)), at least one voltage sensing device adapted to measure avoltage level on the regulated DC voltage buss (V_(DC)), at least onecurrent sensing device adapted to measure a current level on a positiveor a negative output lead of the regulated DC voltage buss coupled tothe corresponding micro-converter (I_(out)), at least one currentsensing device adapted to measure a current level on a negative or apositive input lead of the regulated DC voltage bus coupled to saidcorresponding micro-converter (I_(in)), and a control device that isadapted to calculate a power into the corresponding micro-converter anda power out of said corresponding micro-converter and to compare thepower into the corresponding micro-converter to the power out of saidcorresponding micro-converter; an inverter that is coupled to thevoltage buss and that is structured and arranged to provide power to agrid or to a DC load, the control system being structured and arrangedto control each string of photovoltaic modules.
 14. The control systemas recited in claim 13, wherein the control device of eachmicro-converter is further adapted to disconnect the correspondingstring when the power into the corresponding micro-converter does notequal or substantially equal the power out of said correspondingmicro-converter.
 15. A method of harvesting energy using a photovoltaicpower generating system that includes a power-generating portionincluding plural parallel strings of power-generating photovoltaicmodules, the power-generation portion generating a total power and thepower-generating photovoltaic modules being coupled in series in eachstring of the plural strings, each string of the plural strings and eachof the modules having an output; a plurality of DC/DC micro-converters,each micro-converter of the plurality of DC/DC micro-converters beingcoupled to a regulated DC voltage buss and to the output of acorresponding photovoltaic module or to the output of a correspondingstring of photovoltaic modules, wherein each micro-converter includes afractional power converter; an inverter that is coupled to the regulatedDC voltage buss and to a load; and a control system that is structuredand arranged to control each string of photovoltaic modules, the methodcomprising: connecting each fractional power converter with thepower-generating portion; and injecting one of current and voltagethrough the fractional power converter onto the voltage buss.
 16. Themethod as recited in claim 15, wherein connecting each fractional powerconverter with the power-generating portion includes connecting eachfractional power converter in series with the power-generating portionso that each fractional power converter draws a fraction of the totalpower from the power-generating portion via a parallel input port andinjects a voltage in series with voltage from the power-generatingportion.
 17. The method as recited in claim 15, wherein connecting eachfractional power converter with the power-generating portion includesconnecting each fractional power converter in parallel with thepower-generating portion so that each fractional power converter draws afraction of the total power from the power-generating portion via aseries input port and injects a current in parallel with current fromthe power-generating portion.
 18. The method as recited in claim 15,wherein connecting each fractional power converter with thepower-generating portion includes connecting a bi-directional fractionalpower converter that is capable of injecting power in series or power inparallel with, respectively, voltage or current from thepower-generating portion.
 19. A photovoltaic power generating systemcomprising: a power-generating portion including plural parallel stringsof power-generating photovoltaic modules, the power-generating portiongenerating a total power and the power-generating photovoltaic modulesbeing coupled in series in each string of the plural strings, eachstring of the plural strings and each of the modules having an output; aplurality of DC/DC micro-converters, each micro-converter of theplurality of DC/DC micro-converters being coupled to a regulated DCvoltage buss and to the output of a corresponding photovoltaic module orto the output of a corresponding string of photovoltaic modules, whereineach micro-converter includes: a fractional power converter that isstructured and arranged to inject one of current and voltage,respectively, in parallel with current from the power-generating portionand in series with voltage from the power-generating portion, and aninput port by which the fractional power converter draws a fraction ofthe total power from the power-generating portion; an inverter that iscoupled to the regulated DC voltage buss and to a load; and a controlsystem that is structured and arranged to control each string ofphotovoltaic modules.
 20. A photovoltaic power generating system asrecited in claim 19, wherein the fractional power converter is adaptedto be bi-directional to be capable of injecting power in series withvoltage from the power-generating portion or in parallel with thecurrent from the power-generating portion.